U.S. Pat. No. 4,491,016 discloses a method and apparaus for measuring a pressure as specified above, using a piston which is maintained contact-free within a cylinder by hydrostatic or pneumatic fluid supplied under pressure between the piston and the inner surface of the cylinder.
One problem encountered with such a measuring apparatus is the leaking of hydrostatic or pneumatic fluid into a measuring chamber confined at one side by the face wall of the piston and containing the gas or liquid under the pressure to be measured. This is particularly detrimental when a liquid is used as a hydrostatic fluid and the pressure of a gaseous medium is to be measured. In this case, the hydrostatic liquid will slowly but continuously rise in the measuring chamber and even fill the supply line for the gaseous medium. According to the embodiment of FIG. 5 of U.S. Pat. No. 4,491,016 a resilient separating element is disposed in the measuring apparatus such that only one side of the separating element is connected to a feed-line for the fluid to be measured and the other side of the separating element is in connection with a feed-line to the measuring chamber, and both are filled with the hydrostatic liquid. The separating element is held in a neutral position under equilibrium forces by monitoring means which controls a valve for releasing excess hydrostatic liquid. Such a design needs quite a number of additional elements for controlling a constant volume of the hydrostatic liquid and a separating element for separating the latter from the fluid to be measured. Furthermore, since the hydrostatic liquid is used as a pressure transmission means to the piston it would be inappropriate to use a pneumatic fluid instead of a hydrostatic liquid as a separating agent. Even using a hydrostatic liquid as a transmission means leads to measuring errors due to pressure, temperature, and/or viscosity changes in the hydrostatic liquid.
U.S. Pat. No. 4,541 285 relates to an improvement of the aforementioned apparatus of U.S. Pat. No. 4,491,016, disclosing, a method and apparatus for measuring a pressure of a liquid in a closed storage tank. A measuring piston is guided in a cylinder and is acted upon on one side by a pressure dependent on the pressure to be measured, the piston being in communication with a force measuring device. In order to compensate for the pressure exerted by gas and/or vapor in the interior space of the tank, that side of the piston opposite to said one side is acted upon by this pressure. This is achieved by enclosing the cylinder and the piston in a container filled with a blocking gas, such as nitrogen. The pressure of the blocking gas in the container is controlled by a regulater, operation of which is controlled by sensing means for sensing the gas and/or vapor pressure in the interior of the tank. As a result, relativly complex equipment is necessary to determine that gas and/or vapor pressure and to compensate for it.